The present invention relates to a probe for physiological tonometry or pressure measurement in a human or animal body, the probe comprising a probe head, a line such as a catheter, and a pressure transducer in the probe head. For measuring pressure in the body, the probe head with the pressure transducer is disposed at the measuring site, with the probe head being adapted to be connected to an analyzer and display unit via the line.
Probes of this kind have been known for a long time and are employed particularly for intra-cranial pressure measurement. There, the probe consists of a probe head including an electric pressure transducer. For tonometric measurement, the probe head with the pressure transducer is arranged on the measuring site. The probe head is then connected via a catheter including electric connecting leads to an analyzer and display unit by means of a coupler such as a connector. The pressure measured by the probe or detected by the pressure transducer included in the probe head is then displayed in the display unit.
Such probes may be used mainly to measure the epidural pressurexe2x80x94in this case the probe is located between the cranial bones and the Dura materxe2x80x94the subdural pressurexe2x80x94here the probe is located between the Dura mater and the brain substancexe2x80x94the parenchymatous pressurexe2x80x94for this measurement the probe is located in the brain substancexe2x80x94or the ventricular pressurexe2x80x94in which case the probe is located in a ventricle. All of these measurements are consistently measurements of a relative pressure, which means that the pressure is measured relative to the environment.
The pressure transducer consists, for instance, of a piezo crystal mounted on the distal end of the catheter in the probe head. The catheter may comprise a silicon tube or a tube made of another biologically compatible material, for example, and includes electric leads disposed inside. The catheter is adapted for connection via a coupler to the analyzer and display unit.
Resistance strain gauges or capacitive pressure transducers may also be employed as an alternative to the piezo crystal. The electric pressure transducer is designed as a bridge circuit, e.g. in the form of a Wheatstone bridge, in order to increase the sensitivity. A current or voltage source outside the probe is provided for supplying the bridge circuit or the pressure transducer with electric power. The leads are connected to a voltage source and the pressure transducer.
For tonometric application, the probe is guided to the measuring site in the body. Then the catheter is connected to the analyzer and display unit that serves equally as power or voltage source, i.e. a source of energy, for the probe. The energy and equally the electric signals are transmitted via the electric leads contained in the catheter tube. The measurement of a relative pressure requires the storage of the electric value of the bridge voltage at zero pressure. To this end, null balancing must be carried out.
The crucial point in the known probes resides, however, in the aspect that these probes are only able to perform a certain number of pressure measurements and must then be recalibrated. As a result of the necessary sterilization cycles, the users are therefore required to record a log of the number of the operating applications of the probe. When a predetermined number of operating applications of the probexe2x80x94i.e. of pressure measurementsxe2x80x94is reached, the probe must be returned to the manufacturer for recalibration.
This method of recording the performed operating applications is, however, extremely expensive and, as a rule, does not function in an optimum manner in practical application. As result, probe failure occurs again and again because the probes are used even though the specified number of operating applications has been exceeded. In the everyday routine in a hospital it is impossible to keep records of the number of operating applications. The reliability and safety in operation is then no longer ensured and failure occurs in the implanted condition, which exposes the clinically monitored patient to a substantial risk.
The present invention is therefore based on the problem of improving a probe for physiological pressure measurement in the human or animal body in such a way that a facilitated recording of the operating applications will become possible while the aforementioned disadvantages are avoided.
This problem is solved by providing a counter means which records the number of performed operational applications of the probe.
This problem can also be solved by a controller means, which records a transducer offset value after each autoclaving or sterilization operation performed to sterilize the probe after each use and detects an error condition by detecting abnormal offset values or abnormal offset value changes since a previous autoclaving or sterilization operation.
The invention is based on the finding that counter or controller means are available of such a small size that they can be incorporated into probes so that the probe can record and report its operating applications automatically.
In accordance with the present invention, the probe therefore comprises a counter or controller means recording the operating applications of the probe. In this simple manner it is ensured that each tonometric application will be recorded. Now a record kept by hand is no longer required. Moreover, this provision improves the safety with respect to manipulations of the specified number of operating applications and hence the documentability of possibly asserted warranty claims.
In a preferred embodiment at least one additional counter for counting overload conditions of the pressure transducer is integrated in the unit. It can be set to count overload conditions all the time during autoclaving or sterilization or only during regular operation.
In another preferred embodiment an additional counter for counting autoclaving or sterilization operation is provided. These autoclaving or sterilization operations may be detected by a rise in temperature or pressure.
Another preferred embodiment of the invention comprises an additional timer for measuring operational hours.
The counter means is preferably disposed in that part of the coupler that is connected to the line.
In order to prevent specifically any use of the probe in excess of the specified number of operational uses and hence false measuring results or the failure of the probe during measurement, the counter means renders the pressure transducer and hence the probe inoperative as soon as the specified number of operating applications is reached.
The number of the operating applications and/or the number of applications remaining until the end of service of the probe can be displayed via a display means so that the user can inform himself at any time about the remaining number of operational applications and the number of completed operational uses of the probe. To this end, the display means is integrated, in particular, into that part of the coupler that is connected to the line so that the user can obtain this information from the probe directly.
According to one embodiment of the invention, the probe comprises an electric pressure transducer and an electric counter means. The counter means is configured with temperature-resistant electric components. The counter means triggers a counting operation only when voltage is applied to the probe throughout a defined period. This provision is intended to prevent that the counting operation is triggered when a voltage is applied only briefly. This may be the case, for instance, when the connection with the probe is to be tested. What is to be counted is only the occurrence of an actual application in operation, i.e. when the voltage is applied, for instance, for half an hour at minimum.
To prevent a further use of the probe when the specified number of operating applications has already been reached, the counter means operates a switch that disconnects the power supply of the pressure converter when a specified number of operational applications is reached. The analyzer and display unit recognizes this situation and stops the measurement or no longer permits a further measurement. This prevents a further application of the probe in operation. The user is hence forced in a simple manner to return the probe to the manufacturer for inspection and calibration.
The counter means preferably operates in a decrementing mode for rendering the probe inoperative, i.e. it counts from the specified number down to zero so that when zero is reached the probe is taken out of service. Additionally, the counter means operates continuously in an incrementing mode for recording the total number of operational applications of the probe. The user is thus enabled, on the one hand, to detect the total number of applications of the probe and, on the other hand, to establish the number of potential operating applications remaining up to the point where the probe is rendered inoperative and the probe must be inspected and calibrated by the manufacturer.
To ensure small dimensions of the probe and to widen the range of potential applications even further, the counter means comprises a microprocessor with a memory, with the number of operating applications as well as the remaining number of potential operating applications up to the end of service being stored in this memory.
According to one embodiment of the invention, the display means is constituted by a light-emitting diode that indicates the desired number by a string of flashing signals. With this provision, the smallest display means possible is implemented.
Preferably the number of the specified operational applications of the probe can be set. This number is set in particular when the specified number of operating applications is reached, at which the probe is rendered inoperative, and at the time when the service is resumed after calibration. The setting is carried out by means of appropriate software.
In order to be able to handle maintenance jobs in a facilitated manner the memory stores further data such as manufacturer-specific information such as serial number, customer number, date of delivery, name of the person calibrating the probe, date of last calibration, or the like.
The blood-borne Creutzfeld-Jakob (CFJ) disease has occurred recently to an increasing extent. Insofar it is necessary to use methods for sterilization which operate at temperatures higher than 130xc2x0 C. in order to ensure a complete and safe destruction of the CFJ agents. The probe therefore consists of a biologically compatible material stable in terms of temperature, which allows a sterilization method such as autoclaving/steam sterilization above 130xc2x0 C. As far as is known to date, the CFJ agents are completely destroyed at these temperatures, which permits repeated use of the probe.
A particular field of application of this probe is that of intra-cranial pressure measurement. For this application a small sizexe2x80x94preferably a diameter less than 8 mm is required. With a drop-shaped housing the explantation is simplified significantly.
Another preferred embodiment of the invention comprises a controller which detects an error condition by comparing an offset value by at least on predetermined limit value. So the offset value can be compared for example with an upper limit value and a lower limit value. If it is outside the limit values an error condition occurs.
For improved detection of error conditions a value like the first order derivation which is a measure of change of offset values is calculated. This may be done by subtracting an offset value of a previous offset value and dividing the difference by a value which is either the number of operational/calibration cycles or the time elapsed between the measurement of these values. An error condition may be detected by comparing such a value by at least on predetermined limit value. By this way extreme, abnormal changes of the offset value can be detected.
For further improved detection the previous procedure can be applied to the change of offset values themselves to get a second order derivation. This can also be compared by predetermined limit values.
A method for monitoring lifetime and reliability of pressure probes is described. This method uses a probe head, a line such as a catheter, and a pressure transducer in the probe head. The number of operational applications of the probe are counted.
A method for monitoring lifetime and reliability of pressure probes is described. This method uses a probe head, a line such as a catheter, and a pressure transducer in the probe head. The offset value of the pressure transducer is measured after each sterilization operation or before each new application of the probe. The offset value, changes of offset values, changes of changes of offset values are evaluated to detect error conditions or states of abnormal operation.